ATOMIC WEIGHTS ASTON. 189 



It is somewhat remarkable that notwithstanding the immense 

 amount of research work done on the discharge at low pressure its 

 most obvious phenomena are almost entirely without explanation; 

 modern measurements and other data have merely destroyed the 

 older theories without, as yet, suggesting others to replace them. 

 In discussing positive rays it is of importance to consider the phe- 

 nomena taking place immediately in front of the cathode of the 

 discharge tube. 



The comparatively dimly lit space between the cathode and the 

 bright " negative glow " is named after its discoverer the " Crookes' 

 Dark Space." Its length is roughly inversely proportional to the 

 pressure of the gas in the tube. Its boundary, the edge of the nega- 

 tive glow, is remarkably sharp in most gases, quite amazingly so in 

 pure oxygen. If large plane cathodes are used so that the effect of 

 the glass walls — so far a complete mystery — does not come in, very 

 accurate and consistent measurements of the Crookes' dark space can 

 be made. Working with a very large " guard-ring " cathode, the 

 writer showed in 1907 17 that its length D could be expressed as 



where P is the pressure, i the current density on the surface of the 

 cathode and A and B constants. This expression is fairly exact for 

 ordinary gases, but only approximate for those of the helium group. 18 

 A varies both with the nature of the gas and the metal used as 

 cathode. With the same metal as cathode it is four to five times as 

 big for hydrogen as for oxygen; using the same gas, it is about twice 

 as big with a silver cathode as for one of aluminum. 19 The value 

 of the remarkable constant B, which is independent of the pressure, 

 is also practically unchanged either by the nature of the gas, or the 

 nature or even the shape of the cathode. 



During these investigations it was also demonstrated that in the 

 absence of the positive column, the whole of the potential V required 

 to maintain the discharge, takes place between the cathode and the 

 negative glow, and its relation to the current may be expressed by 

 the equation 



E and F being constants which depend on the nature of the gas and 

 the metal used as cathode. 



Measurements of the distribution of potential in the dark space 

 by a method which appears free from objection 20 show that the field 



"Aston: Proc. Roy. Soc, 79A, 80, 1907. 

 18 Aston and Watson : Ibid., 86A. 168. 1912. 

 "» Aston: Ibid., 87A, 428, 437, 1912. 

 =» Aston : Proc. Boy. Soc, 84A, 526, 1911. 



